1. Field of the Invention
The present invention relates to an image processing apparatus.
2. Description of the Related Art
A conventional image display apparatus disclosed by Nakahigashi et al. in Japanese Patent Application Laid-open No. 2004-342030 improves the contrast of an input signal with components representing values of the three primary colors red, green, and blue by generating a luminance signal from the input signal, generating a gradation-scale correction table from the luminance signal, and multiplying the red, green, and blue component values by a ratio derived from the gradation-scale correction table.
There are two general methods for improving the contrast of a video signal. One general method is to separate the video signal into a luminance component and a color component (e.g., a luminance signal and a pair of color difference signals) and perform a gradation correction on the luminance component. In this method, there is a tendency for increased luminance levels to produce faint colors (colors with low saturation) and reduced luminance levels to produce deep colors (with high saturation). To prevent this occurrence, a gradation correction is sometimes also performed on the color signal (color difference signals), but this may cause the problem of color collapse, in which differences between color gradation levels are lost.
The other general method is to perform the gradation correction on the primary color components of the image signal, normally red, green, and blue, typically using the same gradation-scale correction table independently for each color component. A problem with this method is that it distorts color balance, because the different primary color components of each pixel tend to be corrected by different amounts.
The method disclosed by Nakahigashi et al. combines both of these general methods. By multiplying each of the primary color values of a picture element (pixel) by the same ratio (correction parameter) it avoids distortion of color balance, but since the correction parameter is derived from luminance information, the corrected value of a color component may greatly overshoot the maximum gradation limit, causing severe color collapse.